Fuel burners are used in a variety of applications requiring heat energy for operation. These burner installations range from very simple and small gas dryer and water heater systems, to very large and complex systems operating with a variety of fuels in industrial uses such as in chemical plants or in commercial applications such as heating plants for large buildings or groups of buildings. It is well known of course that burner installations of any size must be carefully managed in order to achieve safe and efficient operation. Where the installation is very small, the control systems have few functions and typically have a simple sequence to bring the system from its standby state where it is waiting for a demand for heat, to its run state where fuel may flow to the combustion chamber at its maximum rate.
In the following description, the term "burner unit" will denote a combustion chamber, a main fuel valve and a pilot fuel valve feeding respectively a main and a pilot burner in the combustion chamber, a pilot fuel igniter, and a blower for providing combustion air to the combustion chamber. Certain types of units will also include a modulating main valve and a damper for regulating flow of combustion air to the combustion chamber which together allow control of the firing rate, or the rate at which fuel is consumed. The valves, igniter, and blower enter their operating states where they provide their respective functions responsive to control signals provided to each by a controller.
The operation of a burner unit is considered to comprise a number of distinct phases of operation. The typical operating sequence provided by the controller for a burner unit as it progresses from its standby mode to its run mode, is first a timed purge period, where the blower signal causes the blower to operate to remove any left-over combustion products from the combustion chamber, followed by a pilot flame ignition period during which the pilot valve signal opens the pilot valve and the igniter signal causes the igniter to operate. After the pilot flame ignition (also referred to as the pilot flame establishment period or PFEP) has been successfully completed then the main valve signal opens the main valve, and the burner unit begins main flame ignition (main flame establishment period or MFEP). The pilot flame ignites the fuel flowing from the main burner, and the burner unit then starts its run mode. The burner unit remains in the run mode until the demand has been satisfied, at which time the controller causes the main valve to close and possibly, the blower to operate for a continued purge period, after which the burner unit returns to its standby mode. In one embodiment, both the PFEP and the MFEP are 10 sec. long.
For certain types of applications, it is very desirable that burner units enter their run mode relatively quickly after it is determined that heat is required. The problem with the operating sequence described above is that for certain types of installations, the purge period is relatively lengthy, perhaps as long as 10 min. or more. Accordingly, the time period between the start of the demand signal and actual start of the run mode may be longer than is desired. On the other hand, it is important that the length of the purge and ignition periods not be shortened in order to assure the removal of any residual fuel leakage and left-over combustion fumes and in order to assure that pilot ignition is successful.
There are certain variations on this starting sequence which have been developed in other burner unit operating contexts. For example, U.S. Pat. No. 4,999,792, which has a common assignee with this application, teaches the use of an intermediate pilot flame stage during the transition from one type of fuel to another in a burner system in which multiple fuels may be used.